Method of making microwave integrated circuits

ABSTRACT

A method of making a multiplicity of microwave integrated circuits is disclosed wherein a thin glass substrate is etched to contain both via holes and large area holes which receive discrete active devices therein. An electrically and thermally conductive carrier is adhered to one surface of the glass substrate with the electrical interconnections and circuit components formed on the opposing surface of the glass substrate. The method enables a multiplicity of microwave circuits to be made and tested by batch processing techniques prior to division into individual circuits.

BACKGROUND OF THE INVENTION

This invention relates to a method of making electronic circuits, and inparticular, to a method of making microwave circuits wherein amultiplicity of circuits are formed and tested in an integral structure,which includes a glass substrate, prior to division into individualcircuits.

In the manufacture of integrated circuits, the production goals of lowcosts and high reproducibility are achieved through the use offabrication processes using photolithographic techniques, whereby amultiplicity of circuits are fabricated in a single manufacturingsequence. This concept, referred to as batch processing, is used in thefabrication of semiconductor devices. The best example of the impact ofbatch processing in achieving low production costs is found in thesilicon integrated circuit industry, where highly complex products areproduced with a remarkably low unit cost.

The use of silicon (Si) integrated circuit technology is limited to lowfrequency applications (generally less than 1 GHz) by the limitedelectron mobility and low intrinsic resistivity of silicon. The lowelectron mobility limits the high frequency performance of the activesemiconductor elements while the low bulk resistivity characteristic ofthe material creates high transmission losses at the higher microwavefrequencies.

Gallium arsenide (GaAs) is presently used as an alternativesemiconductor material for microwave integrated circuits. This materialprovides excellent results in many applications but it also possesseslimitations which preclude its use in other important applications. Themost important limitation of GaAs is the high cost of the basic wafer.GaAs wafers typically cost 20 to 100 times the cost of an equivalentsilicon product. Further, many microwave circuits require large surfaceareas to accommodate the passive element structures and rely onextremely fine conductive pattern resolution during fabrication. As aresult, the equivalent number of circuits available from a single waferis often very low when compared with the number of low frequency siliconcounter-parts obtained from the same size wafer. The materialrequirements lead to an unacceptable cost per circuit. In addition, manycircuit designs require trimming, alignment or other adjustments afterfabrication in order to compensate for variations in the characteristicsof the active microwave devices sited in the individual circuits. Thusfar there is no demonstrated effective technique, similar to the siliconmanufacturing techniques, for accomplishing batch processing with GaAsmircowave integrated circuits.

Electronic components and subsystems are usually comprised of both lowfrequency and high frequency functions. Low frequency functions areususally implemented using silicon devices and integrated circuits. Highfrequency functions in the microwave regime are usually implementedusing GaAs diodes, FETs and integrated circuits. Because mostapplications require several Si and GaAs discrete devices or monolithiccircuits, conventional hybrid technology has been used to incorporatethese functions onto a single or multiple substrates (alumina, duriod,etc.).

To overcome the limitations associated with conventional hybridtechnology, the trend in research has been to increase the level ofintegration and to investigate the integration of different devices on asingle semiconductor substrate. Although impressive progress has beenmade in every area of investigation, near term applications require newmanufacturing techniques for the implementation of ancillary functionsassociated with the utilization of discrete devices and monolithiccircuits fabricated using different semiconductor substratetechnologies. Glass substrate technology is capable of serving the abovefunction with low cost and reproducibility features akin to those ofmonolithic circuit technology. Furthermore, the approach is capable ofincorporating optical devices with low frequency and high frequencyfunctions for use in novel opto-electronic components.

The conventional manufacturing technique for the fabrication of highperformance hybrid microwave integrated circuits (HMIC) utilizes a lowloss dielectric substrate for the placement of passive circuit elementsin combination with packaged or unpackaged semiconductor chips thereon.The most commonly used substrate materials are alumina (Al₂ O₃),beryllia (BeO) and fused silica (SiO₂). Most HMIC's utilize discretechips for the capacitors as well as transistors. Hybrid circuits exhibitexcellent performance characteristics and a high degree of flexibilitybut also require substantial labor for assembly. The high labor contentleads to high fabrication cost and introduces unpredictable variationsin component placement and bonding. These variations in assembly degradethe circuit's performance and highly skilled technicians are required totune the rf circuit in order to attain consistent performance standards.

In the development of an improved low cost HMIC manufacturing process,the use of less expensive dielectric substrate materials which do notcompromise circuit performance need to be considered. The fabricationprocess must allow passive circuit elements and their interconnectionsto be formed by batch processing techniques with high accuracy.Subsequent assembly and test procedures should be compatible withautomation. The inability of present HMIC manufacturing techniques toutilize low cost substrates in an accurate and automated batchprocessing manufacturing sequence has resulted in the high cost ofhybrid microwave integrated circuits. Especially when the cost iscompared to that of silicon circuits fabricated by batch processing andtesting.

Important requirements to be addressed in the fabrication of microwaveintegrated circuits are as follows: the substrate must have low lossmicrowave transmission characteristics; the conductive ground plane forthe circuit must be accessible through short distances to provide lowparasitic inductance to ground; the substrate must exhibit a smoothsurface finish in order to provide a base for the fabrication of largearea passive components using thin film techniques; and the activesemiconductor devices must be mounted on a good thermal conductor toeffectively provide adequate heat dissipation.

Present techniques for fabricating HMIC circuits utilize typically a15-mil thick alumina substrate with a relative dielectric constant of10. Ground connections are usually provided by small diameter via holeswhich are electroplated to the ground plane formed on the bottom surfaceof the substrate. The via holes are formed in the substrate by the useof a laser drilling. Alternatively, the holes can be drilled or punchedmechanically in the substrate in the "green" state prior to sintering.

Laser drilling techniques generate splatter onto adjacent surface areasand result in the build-up of slag about the periphery of the hole. Theaccuracy attainable with laser drilling of small diameter holes is notacceptable for high density circuit fabrication. In addition, opticallytransparent substrate materials, such as fused silica, are extremelydifficult to drill mechanically. In the case of small diameter holeformation while the substrate is in its "green" state, significanttolerance problems have been encountered as the dimensions and internalsurface change as the material is sintered. Consequently, present holeformation techniques in substrates have resulted in reducing the yieldand increasing the cost of microwave integrated circuits.

A micro-smooth surface is required for the fabrication of thin filmcapacitors. Since alumina and beryllia are polycrystaline materials withmany grain boundaries and other surface defects, to produce a smoothsurface, it is necessary to apply an amorphous glaze to the surface ofthe substrate. However, this approach has limited use due to thecomplexity of the process and difficulty in controlling the electricalcharacteristics of the resulting multi-layer dielectric substrate.

The need for a thermally conductive mounting surface for the unpackagedactive devices in an HMIC usually requires attachment of activesemiconductor devices on a metallic carrier placed below the dielectricsubstrates. Since the dielectric substrate is usually much thicker thanthe unpackaged active devices, either a machined metallic carrier or aseparate mechanical processing step is necessary to raise thesemiconductor device to the circuitry located on the upper surface ofthe substrate. It is desirable to limit the thickness of the substratein order to reduce parasitic ground inductances in the circuit.Typically, common substrates with thicknesses on the order of 0.010inches are very difficult to process due to breakage during handling. Asa result, microwave circuits are manufactured today with specificationsand techniques which trade off the various physical limitations of thematerials against the performance and manufacturing costs.

Accordingly, the present invention addresses a method of manufacturewherein batch processing techniques can be used for the definition andformation of passive structural elements and subsequent assembly andtesting of the microwave integrated circuit can be performed in anautomated fashion prior to the separation of the substrate into amultiplicity of individual circuits. Thus, the heretofore practicedintermediate step of separating the substrate into individual circuitsprior to location and attachment of the die containing the activesemiconductor elements is eliminated.

Furthermore, the present method is well-suited for the use of thinsubstrate materials. In the prior art it is recognized that thethickness of the substrate is dictated in part by the dielectricconstant of the substrate material. As the substrate thicknessincreases, the likelihood that present processing steps utilized forhole formation will result in damage to the substrate surface areaproximate to the hole also increases since the time required to effecthole formation increases correspondingly. Also, the time in whichundesired lateral effects can take place on the surface of the substrateincreases. Accordingly, the present invention is directed to a novelmethod of making microwave integrated circuits, wherein a relativelythin substrate possessing a low dielectric constant is utilized in orderto reduce the processing time required for hole formation. Thisinvention further includes the step of chemically etching holes in thesubstrate to substantially reduce the undesired effects of presentmanufacturing techniques.

SUMMARY OF THE INVENTION

This invention relates to a method of making a multiplicity ofindividual microwave integrated circuits wherein the fabrication andassembly of integrated circuits is performed with batch processingtechniques through the final testing step. The method employs a glasssubstrate which serves both as a low loss dielectric material for thetransmission of energy at microwave frequencies and as the carrier forcircuit components.

The present invention utilizes a glass substrate having first and secondopposing surfaces with at least one of the surfaces having selectedareas etched therein to form a multiplicity of openings extendingthrough the substrate. The openings include small area via holes, andlarge area openings for the placement of active devices. The via holesare plated through to form ground connections between the ground planeformed on one surface and the conductive pattern formed on the opposingsurface of the substrate. Next, an electrically and thermally conductivecarrier is adhered to the opposing surface of the glass substrate. Byattaching the glass substrate to the carrier early in the sequence ofprocess steps, the following steps are performed on a mechanicallyrugged composite material thereby oermitting a relatively thin glasssubstrate to be used without significant breakage during circuitfabrication. Then, a multiplicity of electrical circuit components andan interconnecting conductive pattern is formed on at least one surfaceof the substrate. The provision of a functionally complete and ruggedsubstrate prior to the attachment of active devices, wire bonding andtesting, in the present method, eliminates the need to separate theindividual circuits and subsequent mounting onto individual carriers forfurther processing. Next, a multiplicity of active devices are sited inthe larger area openings of the glass substrate. The active devices aredie-attached to the conductive carrier so as to be in heat-transferrelationship thereto rather than being sited on the upper surface of thesubstrate.

When the active devices are located in their respective openings,electrical interconnections between the electrodes of the active devicesand the appropriate adjacent conductive pattern are formed using wirebonding techniques. Upon interconnection the circuits are fullyfunctional and can be tested. Then, the substrate and carrier aredivided into a multiplicity of individual microwave integrated circuits,each of which contains at least one active device or integrated circuitpositioned therein. Thus, the entire fabrication process takes placewith the large area substrate being maintained as an integral unitthroughout the entire process. No subdivision takes place as heretoforepracticed in the manufacture of conventional microwave integratedcircuits. As a result, the fabrication process which is the subject ofthe present invention, eliminates operator-dependent steps therebyincreasing yield, reducing cost and decreasing overall fabrication time.

Further features and advantages of the invention will become morereadily apparent from the following detailed description of a specificembodiment thereof when taken in conjunction with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing major steps in the fabricationsequence as utilized in the present embodiment of the present invention;

FIG. 2 is a perspective view of a portion of an active device situatedin the substrate in accordance with the present invention; and

FIG. 3 is a cross-sectional view across line 3--3 of FIG. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The fabrication method, which is the subject of the present invention,is shown in the block diagram in FIG. 1. The fabrication sequence iscomprised of steps represented by blocks through 21.

The fabrication of a microwave integrated circuit begins with asubstrate with a smooth surface. In the practice of the presentinvention, the substrate is a glass substrate, preferably a borosilicateglass characterized by a dielectric loss tangent of less than 0.5% at 1GH_(z) and having a thickness within the range of 4 to 12 mils.Borosilicate glass has a relatively low dielectric constant and iscommercially available from a number of manufacturers, for example the7070 glass manufactured by Corning Glass Works which is capable ofproducing a micro-smooth surface "as fired" without mechanical lapping.The glass substrate provides a micro-smooth surface for thin filmcapacitor fabrication and possesses sufficient structural integrity towithstand the rigors of the subsequent limited processing techniques. Inparticular, the substrate is then masked, etched to form the holepattern and then subjected to a plating process on one side thereof.

Initially, the thin glass substrate has a mask formed thereon configuredin accordance with the topographical features desired for subsequentsteps. Typically, the mask is formed by sputtering a metallic filmthereon which is then subject to selective removal thereby forming thedesired pattern. The following step of the process is directed to theremoval of material from the unmasked portions of the substrate by theetching of holes therethrough. The etching process is directed toetching the smaller diameter via holes, which are later plated throughto provide interconnection between conductive members on either side ofthe substrate, and the large diameter holes, which will accommodate theplacement of active devices therein in a later processing step. Inpractice, the etching is provided by a wet-chemistry step utilizing ahydroflouric acid etchant, preferably etching from both sides of thesubstrate. The masking of both sides of the substrate shortens the timerequired for hole formation and thus reduces the lateral etching of thesidewalls in the opening due to undercut of the masking layer. Uponcompletion of the etching process, the mask is removed from bothsurfaces of the substrate, as noted in block 13.

The underside of the substrate is then electroplated with gold to formthe ground plane for microstrip transmission lines. The thickness of theelectroplated gold layer is typically on the order of 1-2 microns. Thislayer provides both good electrical and mechanical contact with thecarrier, which will be affixed to the back side of the substrate. Inblock 15, the carrier is attached to the substrate. In the presentembodiment the carrier is a highly doped Por N type silicon wafer withresistivities in the range of 0.005 to 0.15 ohm-cm and thickness of10-30 mils. The carrier is selected to match the expansioncharacteristic of the glass over the temperature range of interest andexhibits high thermal and electrical conductivity. The carrier alsoprovides mechanical strength to reduce breakage of the thinner glasssubstrate in subsequent process steps. Other carrier materials foundsuitable for use in the present invention composites which exhibit goodthermal match to semiconductor materials, high electrical and thermalconductivities. Attachment of the substrate to the carrier is obtainedby coating the carrier with a thin layer of gold and placing the carrierin contact with the gold on the substrate. The substrate is bonded tothe carrier by heating the assembly to 370° C., which is the AuSieutetic temperature.

Following the attachment of the plated surface of the substrate to thecarrier, additional coating and masking steps are carried out on theexposed surface of the substrate to form the conductive pattern for thecircuits and passive components. Provisions are made to include thinfilm resistors, capacitors and inductors. Typically, a sequence ofconventional photolithographic steps is utilized to form the componentsand their interconnecting pattern. Upon completion of the formation ofpassive components and their interconnections, the circuit is probedusing automatic test equipment to insure that the components exhibit thedesired electrical characteristics.

Upon completion of the tests, active chips, typically gallium arsenidedevices, are placed within the large area openings in the substrate andattached to the conductive carrier. Die attachment can be performedusing different techniques. The base of the active device is adhered tothe conductive layer on the carrier by the use of a conductive epoxy,for example, a silver-filled epoxy such as "36-2" manufactured byAblestick Laboratories may be used, to insure good electrical contactand thermal contact with the carrier. Alternatively, a suitable solderalloy such as Au-Sn may also be used. Upon die-attach of the activedevices, thermo-compression bonding is used to connect the devices tosuitable bonding pads which are part of the circuitry fabricated on thesubstrate. At this point, the circuits can be tested using automatictesting equipment. The electrical tests are designed in accordance withrf specifications. Following the recording of the test results, thelarge area substrate is divided into a multiplicity of individualmicrowave integrated circuits. Because the relative positions of theindividual circuits formed on this large area glass substrate aremaintained through the entire manufacturing process, wire bonding andtest can be readily automated using existing automatic equipment.

In contrast, conventional processes utilized in the manufacture ofhybrid microwave integrated circuits require the division of substratesprior to the placement and attachment of the die in the circuit. As aresult, each microwave integrated circuit is formed as an individualunit after the generation of the circuit passive components andconductive pattern. After division of the substrate in the priorprocesses, each individual circuit, less its active element, is thenattached to a carrier. Then, the die attachment and wire bonding stepsare performed on the individual units. For those reasons, the presentinvention provides a processing method for the manufacture of microwaveintegrated circuits at reduced cost and higher yield.

One embodiment of a microwave circuit constructed in accordance with themethod which is the subject invention is shown in the partial sectionview of FIG. 2 wherein a semiconductor die 22 containing an activedevice, for example, a gallium arsenide transistor and associatedcircuitry, is shown located in a large area opening of a borosilicateglass substrate 24 having a thickness of 10 mils. As noted from FIG. 3,the active device 22 is attached to the ground plane 26 formed on theupper surface of carrier 25. Wire bonds 30 extend from adjacent bondingpads and passive components across the space between the vertical wallof the substrate and the active device connecting appropriate bondingpads located on the upper surface of the unpackaged active device. Inthe embodiment shown, the carrier 25 is highly doped silicon and theground plane 26 is a gold layer formed thereon and upon whichborosilicate glass substrate 24 is adhered. The sidewalls 40 of theopening 23, which is chemically etched into the glass substrate 24, areshown slanted inwardly toward the center of the hole as a result ofbeing chemically etched from both sides of the substrate.

A via hole 41, having a relatively small diameter, is shown in FIG. 3and extends through the glass substrate 24. Both the large diameter hole23 which accommodates the active device 22 and the small via hole 41 areprovided with what is termed wrap-around metalization to bring thecircuit ground to the top surface of the integrated circuit. A capacitor33 is formed on the substrate surface and includes bottom electrode 36which is connected to the metal layer extending downwardly throughopening 23, dielectric layer 35 formed thereon and top electrode 34which serves as a bonding pad not only for the wire bonds 30 but alsofor the interconnections 37 to adjacent bonding pads 38 and 39. Atypical bonding pad 42 for interconnection to other circuit areas isshown in FIG. 2.

In the practice of the present method, glass substrate 24 is masked,etched and the mask removed as an integral large area substrate for amultiplicity of individual microwave integrated circuits. Eachindividual microwave integrated circuit formed thereon may include morethan one active device and associated via holes. The layout andtopography of the individual circuits comprises no part of the presentinvention since it is unique to the particular application and does notalter the process technology. Upon completion of the formation of thecircuits on the substrate upper surface and the adherence of the carrierto the opposing surface, the substrate is divided into individualcircuits utilizing a conventional saw for semiconductor materials. Theindividual microwave integrated circuit can then be incorporated intohigher level assemblies or packaged for individual use.

While the foregoing description has referred to a specific embodiment ofthe invention, it is recognized that many modifications and variationsmay be made therein without departing from the scope of the invention asclaimed.

What is claimed is:
 1. A method of making a multiplicity of individualmicrowave integrated circuits wherein a glass substrate is utilized bothas a dielectric material for the transmission of microwave energy and asa fabrication base for circuit components, said method comprising thefollowing steps:(a) etching selected areas of at least one surface of athin glass substrate having a thickness within the range of 4 to 12 milsand including first and second opposing surfaces to form a multiplicityof openings extending therethrough; (b) adhering an electrically andthermally conductive carrier to said first or second opposing surface ofsaid glass substrate; (c) forming a multiplicity of conductive patternson said at least one surface of the substrate; (d) placing activedevices within selected openings of said multiplicity of opening saidsubstrate; (e) affixing said active devices to said conductive carrierin heat transfer relationship thereto; (f) forming electricalinterconnections between each of said active devices and an adjacentportion of the conductive pattern located on said at least one surfaceof the substrate; and (g) dividing said substrate and carrier into amultiplicity of individual integrated circuits containing active devicespositioned therein.
 2. The method of claim 1 wherein said initial stepcomprises:(a) etching selected areas of said thin glass substrate toform a multiplicity of small and large diameter openings, said largediameter openings receiving active devices therein.
 3. The method ofclaim 2 further comprising the steps of:(a) locating a mask on said onesurface of the glass substrate to define a multiplicity of masked areasthereon; and (b) chemically etching said unmasked areas to form smalland large diameter openings extending through the glass substrate priorto adhering said carrier to the opposing surface of said substrate. 4.The method of claim 3 further comprising the steps of locating masks onboth surfaces of said substrate and then chemically etching unmaskedareas on both surfaces of said glass substrate.
 5. The method of claim 4wherein the step of adhering a conductive carrier to said layercomprises: adhering a metal composite carrier to said opposing layer toform a structure for subsequent fabrication steps.
 6. A method of makinga multiplicity of individual microwave integrated circuits wherein aglass substrate is utilized both as a dielectric material for thetransmission of microwave energy and as a fabrication base for circuitcomponents, said method comprising the following steps:(a) etchingselected areas of at least one surface of a glass substrate having firstand second opposing surfaces to form a multiplicity of openingsextending therethrough; (b) plating a conductive layer on said first orsecond opposing surface of said glass substrate; (c) adhering anelectrically and thermally conductive carrier to said layer; (d) forminga multiplicity of conductive patterns on said at least one surface ofthe substrate; (e) placing active devices within selected openings ofsaid multiplicity of openings in said substrate; (f) affixing saidactive devices to said conductive carrier in heat transfer relationshipthereto; (g) forming electrical interconnections between each of saidactive devices and an adjacent portion of the conductive pattern locatedon said at least one surface of the substrate; and (h) dividing saidsubstrate and carrier into a multiplicity of individual integratedcircuits containing active devices positioned therein.
 7. The method ofclaim 6 wherein said initial step comprises:(a) etching selected areasof at least one surface of a thin glass substrate having a thicknesswithin the range of 4 to 12 mils to form a multiplicity of openingsextending therethrough.
 8. The method of claim 6 wherein the step ofadhering a conductive carrier to said layer comprises:(a) placing acarrier formed of a semiconductor material and having a thickness atleast as large as the glass substrate in contact with said layer; and(b) adhering said carrier to said layer and glass substrate.
 9. Themethod of claim 6 wherein the step of adhering a conductive carrier tosaid layer comprises:(a) plating a metallic layer on said opposinglayer; and (b) adhering a highly-doped silicon carrier, a metalcomposite or other carrier material thereto to form a structure forsubsequent fabrication steps.